1. Field of the Invention
The present invention relates to a method and apparatus for screening for retinopathy, such as diabetic retinopathy, and in particular to a method and apparatus for providing an objective indication of the severity of retinal disease and disease features, such as diabetic retinopathy and retinal ischemia, using pupillometry and a plurality of light stimuli.
2. Description of Related Art
There is an epidemic of diabetes in this country. Diabetic retinopathy, which is a pathological disorder of the retina caused by diabetes, is a major health threat to diabetics. It is estimated that more than 80% of diabetics will develop diabetic retinopathy to some extent, with a large percentage requiring treatment. The most common cause of vision loss in diabetics is the failure to recognize and treat diabetic retinopathy. Thus, effective screening for diabetic retinopathy is essential for diabetics to maintain visual function and quality of life.
Diabetic retinopathy is the result of retinal capillary damage caused by diabetes. The progressive loss of retinal capillaries leads to areas of retinal ischemia, which is a decrease in blood supply to the retina, primarily in the midperipheral portion, and to a lesser extent in the central portion, of the retina. Retinal ischemia promotes angiogenesis, also called neovascularization, which is the growth of new blood vessels in the retina. When uncontrolled, angiogenesis can cause damage to normal retinal tissues because the new vessels are fragile and hemorrhage easily. In addition, ischemic changes promote increased permeability of the retinal blood vessels, which leads to a swelling and thickening of the central portion of the retina, a condition called macular edema (the macula is an area near the center of the retina), and reduced visual function. Midperipheral retinal ischemia, while primarily seen in diabetic retinopathy, is also seen in a number of other conditions, including sickle cell disease, retinitis pigmentosa, Eales disease, and radiation retinopathy, among others. An ischemic retina has reduced sensitivity to light in portions thereof, which is why diabetics with advanced forms of the disease are often night blind and have reduced peripheral vision.
In screening for diabetic retinopathy, a clinician thus needs to look for signs of both macular edema and midperipheral retinal ischemia. The extent of retinal thickening in the macula (macular edema) can be a quantitatively assessed using an existing technology known as optical coherence tomography (OCT). OCT provides actual dimensions of the thickness of the retina in the central 6 mm thereof, and thus can be used to detect macular edema that might adversely effect vision.
Unlike macular edema, there is currently no known quantitative screening method for retinal ischemia. Instead, current screening methods for retinal ischemia involve the subjective clinical observation of various physiological conditions including nerve fiber layer infarcts, arteriolar narrowing, venous bleeding and actual neovascularization. Studies have shown that clinical expertise has a great deal to do with the ability to recognize severe preproliferative diabetic retinopathy, and in particular retinal ischemia, through clinical examinations. This is due, in part, to the fact that the midperipheral portion of the retina is poorly visualized by routine clinical exams and most retinal photography systems. As a result, screening programs based on clinical examinations have a high no show rate, and many cases of diabetic retinopathy are not diagnosed until proliferative retinal changes are extensive.
A number of known screening systems have been devised that translate the clinical examination to an imaging effort. In such systems, images of the retina are transmitted to a reading center where they are examined by trained clinicians to assess disease features. Such systems have several disadvantages, including delays in determining patient status, since images must be transmitted to and reviewed by a remote clinician (such delays create a potential for loss of follow-up with the patient), the high manpower costs required to implement the systems, the limitations imposed by media opacities that affect the quality of the transmitted images, and the fact that such systems still rely on subjective clinical examinations and, as a result, present the same problems described above.
The most direct way to detect retinal ischemia is with a test known as a fluorescein angiogram. A fluorescein angiogram, however, is an invasive procedure that requires dilation of the eye, the injection of dye into the patient and highly specialized photography. As a result, fluorescein angiograms present the potential for numerous complications. In addition, reduced retinal light sensitivity, such as is caused by retinal ischemia, can be assessed through known techniques such as visual field testing and multifocal electrophysiology. These techniques have several disadvantages associated with them. In particular, visual field testing is subjective, time consuming, and relies on the active and accurate participation by the patient, and multi-focal electrophysiology is costly, time consuming, and requires dilation and significant patient cooperation.
Thus, there is a need for a screening approach for retinal disease and associated features, such as those that cause reduced light sensitivity in portions of the retina as is the case with retinal ischemia, that is objective, quantitative, requires minimal cooperation of the patient, requires limited technician skills, and that can provide an immediate clinical assessment.